Adjust config of nic_hotplug to work properly on s390x

On s390x rtl8139 and e1000 are not supported (similarly to ppc64) and
the virtio-net is called virtio-net-ccw.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/nic_hotplug.cfg

@@ -9,7 +9,7 @@
     run_dhclient = no
     variants:
         - nic_8139:
-            no ppc64 ppc64le 
+            no ppc64, ppc64le, s390x
             pci_model = rtl8139
             match_string = "8139"
         - nic_virtio:
@@ -21,7 +21,7 @@
             s390x:
                 pci_model = virtio-net-ccw
         - nic_e1000:
-            no ppc64 ppc64le
+            no ppc64, ppc64le, s390x
             pci_model = e1000
             match_string = "Gigabit Ethernet Controller"
     variants: